Safety fuse for electric circuits



Nov. 24, 1942.

J. A. M. VAN LIEMPT 2,302,820 SAFETY FUSE FOR ELECTRIC CIRCUITS Filed July 51, 1939 3 4 Afl/ 13 (as/v 7-0/7 J RMJ/ZIJLL a'em bz Patented Nov. 24, 1942 SAFETY FUSE FOR ELECTRIC CIRCUITS Johannes Antonius Mari a van Liempt, Eindhoven,

Netherlands, assignor, by mesne assignments, to Hartford National Bank and Trust Company, Hartford, Conn., trustee Application July 31, 1939, Serial No. 287,664 In the Netherlands September 1, 1938 13 Claims.

It is frequently desirable to have at ones disposal a fuse which can repeatedly sustain a current intensity exceeding the limiting current of the fuse during a comparatively long period of time, that is to say a period of time longer than the heating-up period of the fuse. In addition it is necessary that with the same current intensity the fuse in question should break down upon being loaded once during a period of time which but slightly exceeds the above-mentioned long period of time. Besides, such a fuse generally has to satisfy the further requirement that it should be able continuously to sustain a current, for example, of the order of magnitude of the limiting current, for a very long time.

By the heating-up period I mean the lapse of time for a bare fuse at a definite current intensity between the moment at which the current starts passing through the fuse, which is initially at room temperature, and the interruption of the circuit when the fuse breaks down, for example due to melting or oxidation. It may be determined with the aid of an oscillograph, for example a cathode ray oscillograph. For currents which exceed the limiting current at least twice and with which heating-up is effected adiabatically, the heating-up period may be calculated from the formula of Meyer:

in which t is the time of seconds, q the crosssection of the fuse in square-cms., I the shortcircuit current in amperes and C a constant for the material of the fuse. In the present speciconductor but which, in the case of the passage of an excessive current, undergoes, due to the Joulean heat developed, such a change that the fication the term limiting current is always to be understood to mean the limiting current of the bare fuse; by repeatedly I mean at least 500 times.

The usual fuse constructions wherein a fusewire is stretched in air or in another gaseous atmosphere cannot satisfy the said requirements. Better results are obtained with fuses embedded in sand, but in this .case it is not possible to bring the above-mentioned two periods of time sufficiently close to one another.

The fuse according to the invention satisfies these requirements and affords still further advantages over the fuse embedded in sand, as will hereinafter be set out in detail.

The fuse according to the invention is characterised in that it comprises a conductor embedded in a solid mass which is a poor conductor or a non-conductor of electricity and which has a cooling function with regard to the conductor is interrupted. Owing to the fact that the solid mass, which may be a semi-conductor, has a cooling function with regard to the conductor, it is possible to load the fuse for a comparatively long time with a current intensity exceeding the limiting current without breakdown of the fuse. The Joulean heat produced in this case is dissipated by the mass. However, if an excessive passage of energy through the fuse occurs the amount of Joulean heat developed becomes so large that the mass in which the conductor is embedded undergoes a change with the result that the conductor is interrupted.

The term at least poorly electrical insulating material as used "in the claims is to be understood to include a non-conductive material, a poorly-conductive material and semi-conductive material.

This interruption may take place in various ways. It is possible to constitute the mass by a material which springs apart in the case of an excessive passage of current so that the conductor is exposed, at least locally, to the atmosphere surrounding the fuse, e. g. air, and no longer has the opportunity of delivering the Joulean heat to the mass, dueto which at least local overheating of the conductor occurs with the result that the latter melts or is consumed by oxidation. The mass surrounding the conductor may alternatively be formed so as to become plastic in the case of an excessive passage of currentso that, due to the weight of the material that has become plastic, the conductor is torn apart. It will be evident that numerous other ways are possible, inter alia the combination of the two above-mentioned effects.

The conductor and the mass surrounding are dimensioned in accordance with the requirement the fuse has to comply with. Thus, for example, it is possible, in accordance with the invention, to effect this in such manner that the combination repeatedly withstands a current intensity which amounts to three times the limiting current during a period of time t and only once during a period of time 10-25 t. It is possible, for example, to give the conductor and the mass such dimensions that they repeatedly withstand a current intensity which amounts to three times the limiting current for three seconds and only once for seconds. Such values cannot be attained with the known fuse-constructions.

The fuse according to the invention may be intensities use is preferably made, in order to give the conductor the largest possible dimensions, of a material having a high specific resistance, e. g. constantan wire.

Furthermore, it is advisable, according to the invention, to provide the conductor with a layer of precious metal such, for example, as platinum. It is thus ensured that the mass in which the conductor is embedded does not attack the material of the conductor in the course of time and, besides, that any air bubble which may be present between the mass and the conductor exerts no harmful effect on the conductor, as a result of which the latter could be weakened.

A suitable material for the fuse is an alloy of Fe and Ni having a coefficient of transverse expansion which is equal to that of ordinary glass. One embodiment of the invention which may be mentioned, by way of example, is a platinised Fe-Ni wire having a thickness of 28 microns and a length of about 2 cms. and provided on either side with thicker copper wires. This unit is sealed into glass whose coefficient of expansion is equal to that of the fuse-wire, viz. into about 50 to 100 mgs. of glass per cm. of Fe-Ni wire. At 600 milliamperes such a fuse breaks down after about 1 minute whilst it may be exposed many hundreds of times to the same current for 2 seconds. The limiting current of the fuse of this bare platinised Fe-Ni wire is about 250 milliamperes. Such fuses may be more easily manufactured than sandembedded fuses and, moreover, if a transparent mass is chosen, microscopic inspection of the fuse is possible. The combination may be kept small so that it can easily be incorporated in apparatus or lamp caps, if necessary embedded in gypsum. It is not necessary to construct the fuse in a linear form since it may be given, for example, the shape of a V or an N, dependent on the shape of the space, for example in a lamp, discharge tube or other device, in which the fuse has to be arranged.

If it is desired to utilise, instead of ordinary glass, hard glass (having a smaller coeflicient of expansion) a Fe-Ni alloy containing about 40% Ni may be utilised as the conductor, which may be coated, if desired with a thin layer of platinum.

The operation of sand-embedded fuses depends on the position of the fuse (horizontal or vertical), the more or less compressed condition of the sand which may be varied by shaking, etc., but this is not the case with the fuse according to the invention.

The invention will be explained more fully with reference to the accompanying drawing, in which Figure 1 is a sectionized side view of a fuse according to the invention,

Fig. 2 is a sectionized side view of the fuse of Fig. 1 after the conductor thereof has been interrupted,

Fig. 3 is a partly-sectionized side view of a fuse according to another embodiment of the invention,

Fig. 4 is a partly-sectionized side view of the fuse of Fig. 3 after the conductor thereof has been interrupted,

Fig. 5 is a partly-sectionized side view of a fuse according to another embodiment of the invention,

Fig. 6 is a partly-sectionized side view of the fuse of Fig. 5 after the conductor thereof has been interrupted,

Fig. 7 is a sectionized side view of a fuse according to another embodiment of the invention, and

8. 8 is a sectionized side view of the fuse of Fig.7 after the conductor thereof has been interrupted.

Figs. 1 and 2 represent one form of construction of fuse according to the invention wherein a fuse-conductor 3 is secured to current-supply wires I and 2 and embedded in a solid mass 4. The mass 4, which is constituted by a material which is a poor conductor or non-conductor of electricity such as glass, has under ordinary conditions a cooling function with regard to the conductor 3. When an excessive amount of energy passes through the conductor 3 the glass 4 gradually becomes plastic, owing to which it assumes the shape indicated in Fig. 2. Due to the weight of the plastic mass the conductor 3 is broken so that the circuit in which the fuse is included is interrupted. When the mass in which the conductor is embedded consists of transparent material such as glass it is possible to submit the conductor after manufacture to microscopic inspection, which is of great importance especially for comparatively thin conductors.

Figs. 3 and 4 represent another form of construction of the fuse according to the invention, wherein the current-supply wires 5 and 6 are connected to a fuse-wire I which is embedded in a mass 8 of some kind of artificial resin. The conductor 5 has connected to it one of the ends of a helical spring 9 whose other end is secured in the mass 8, this helical spring being mounted with a certain preliminary tension. Upon the passage of an excessive amount of energy through the fuse-wire 'I the mass 8 becomes plastic owing to which the spring 9 draws the mass 8 and the conductor 5 asunder with the result that the conductor I breaks and the circuit in which the fuse is included is interrupted.

In the form of construction according to Figs. 5 and 6 the mass ID in which the fuse-conductor H is incorporated consists of a kind of glass which, upon the passage of an excessive amount of energy through the conductor, cracks due to the Joulean heat developed therein. The crack I! extends completely or partly round the mass I I so that the conductor ll comes locally into contact with the surrounding medium, e. g., air, and besides, due to the local absence of the mass, the heat is no longer dissipated with the result that the conductor oxidizes very rapidly and burns out.

Figs. 7 and 8 represent a form of construction wherein the fuse is formed in the shape of a U. Upon the passage of an excesive amount of energy through the fuse-conductor I! the mass I4 in which the conductor is embedded becomes somewhat plastic whilst, due to the weight of the plastic mass, the conductor I3 is broken.

What I claim is:

l. A fuse for an electric circuit comprising a conductor, and means actuated by the Joulean heat developed to cause an interruption in said conductor when an excessive current passes therethrough, said means comprising a solid mass of an at least poorly electrical insulating material material surrounding the active portion of said conductor, said mass when heated by the Joulean heat developed in the conductor by the passage of the excessive current being physically changed into a form resulting in the interruption of the conductor, said fuse sustaining repeatedly current impulses of an intensity greater than the limiting current and of a duration t not greater than the heating-up period at said current intensity and sustaining only once current impulses of said intensity and of a duration of about Xt.

3. A fuse for an electric circuit comprising a conductor, and means actuated by the Joulean heat developed to cause an interruption in said conductor when an excessive current passes therethrough, said means comprising a solid mass, of an at least poorly electrical insulating material surrounding the active portion of said conductor, said mass when heated by the Joulean heat developed in the conductor by the passage or the excessive current being physically changed into a form resulting in the interruption of the conductor, said fuse sustaining repeatedly current impulses of an intensity greater than the limiting current and of a duration of about 2 seconds, and sustaining only .once current impulses of the same intensity and of a duration of about 100 seconds.

4. A fuse for an electric circuit comprising a conductor, and means actuated by the Joulean heat developed to cause an interruption in said conductor when an excessive current passes therethrough, said means comprising a solid mass of an at least poorly electrical insulating material surrounding the active'portion of said conductor, said mass when heated by the Joulean heat developed in the conductor by the passage of the excessive current being physically changed into a form resulting in the interruption of the conductor, said fuse sustaining repeatedly current impulses of an intensity greater than the limiting current and of a duration t not greater than the heating-up time at said current intensity, sustaining only once current impulses of the said intensity and of a duration of about 10 t, and sustaining for at least 1000 hours a continuous current of an intensity approximately that of the limiting current.

5. A fuse for an electric circuit comprising a conductor, and means to cause said conductor to fracture by gravity when an excessive current passes therethrough, said means comprising a solid mass of at least poorly electrical insulating material surrounding said conductor and adapted to be changed into a plastic state by the Joulean heat developed in the conductor by the passage of the excessive current.

6. A fuse for an electric circuit comprising a conductor, and means actuated by the Joulean heat developed to cause local overheating and an interruption in said conductor when an .ex-

' cessive current passes therethrough, said means comprising a solid mass of at least poorly electrical insulating material surrounding the active portion of said conductor and adapted to be cracked by the Joulean heat developed in the conductor by the passage of the excessive current,

7. A fuse for an electric circuit comprising a conductor, means actuated by the Joulean heat developed to cause an-interruption in said conductor when an excessive current passes therethrough, said means comprising a solid mass of an at least poorly electrical insulating material surrounding the active portion of said conductor, said mass when heated by the Joulean heat developed in the conductor by the passage of the excessive current being physically changed into a form resulting in the interruption of the conductor and means to aid the interruption of the conductor.

8. A fuse for an electric circuit comprising a conductor, a coating 0! a precious metal on said conductor, and means actuated by the Joulean heat developed to cause an interruption in said conductor when an excessive current passes therethrough, said means comprising a solid mass of an at least poorly electrical insulating material surrounding the active portion of the coated I conductor, said mass when heated by the Joulean heat developed in the conductor by the passage of the excessive current being physically changed into a form resulting in the interruption of the conductor.

9. A fuse for an electric circuit comprising a Fe-Ni wire and means actuated by the Joulean heat developed to cause an interruption in said wire when an excessive current passes therethrough, said means comprising a solid mass of an at least poorly electrical insulating material surrounding the active portion of said wire, said mass when heated by the Joulean heat developed in the conductor by the passage of the excessive current being physically changed into a form resulting in the interruption of the conductor.

10. A fuse for an electric circuit comprising a conductor, and means actuated by the Joulean heat developed to cause an interruption in said conductor when an excessive current passes therethrough, said means comprising a mass of glass surrounding the active portion of said conductor, said mass when heated by the Joulean heat developed in the conductor by the passage of the excessive current being physically changed into a form resulting in the interruption of the conductor.

11. A fuse for an electric circuit comprising a conductor, and means actuated by the Joulean heat developed to cause an interruption in said conductor when an excessive current passes therethrough, said means comprising a solid mass of an artificial resin surrounding the active portion of said conductor, said mass when heated by the Joulean heat developed in the conductor by the passage of the excessive current being physically changed into a form resulting in the interruption of the conductor.

12. A fuse for an electric circuit comprising a conductor, and means actuated by the Joulean heat developed to cause an interruption in said conductor when an excessive current passes there-' through, said means comprising a solid mass of an at least poorly electrical insulating material surrounding the active portion of said conductor, said mass when heated by the Joulean heat developed in the conductor by the passage of the excessive current being physically changed into a form resulting in the interruption of the conductor, the material oi said conductor and said insulating material having substantially the same coefflcients of expansion.

13. A fuse for an electric circuit comprising a conductor, and means actuated by the Joulean heat developed to cause an interruption in said conductor when an excessive current passes therethrough, said means comprising a solid mass of an at least poorly electrical insulating material surrounding the active portion of said conductor and adapted to be changed to a plastic state by the Joulean heat developed in the conductor by the passage of the excessive current, said fuse having a curved shape.

JOHANNES ANTONIUS MARIA vm LIEMPT. 

